Spie Press Book

This tutorial fully explains cathode ray tube (CRT) based displays in a single, easy-to-understand narrative. Detailed explanations and insights into performance properties and safety limits of the various glass melts follow a discussion of the fundamentals. In addition, other topics covered include the architectural differences between color and monochrome, the cathode (electron beam source) as a failure mode for all CRTs, types of cathodes available and their life expectancy. Phosphors, the metrics involved in defining a pixel and how distortions can influence the net results, defining CRT compliance with the DICOM Grayscale Standard Display Function (GSDF), test patterns and how they provide information about display performance, and video cards round out this informative work.

The purpose of this book is to bring a broad spectrum of information related to cathode
ray tube (CRT)-based displays into a single easy-to-understand narrative. It requires no
working knowledge of a television or how one programs a video cassette recorder
(VCR). The starting point of each chapter will be basic information that is followed by
detailed explanations and insight into the design trade offs that influence the image
observed. The sequence of topics follows that in a workshop prepared by the author,
and the chapters may be read in any order. However, the information in each chapter
does build upon the material in the preceding chapters. It should be noted that all
references to a cathode ray tube in this book are to only the glass part giving off light in
a display. They do not include the entire display or monitor with the associated
electronics, a point of confusion at times even within the industry.

All CRTs use glass as a starting point; formulas involved (glass melt) provide the
variations in performance. In the 1950s when big names in television such as RCA,
Sylvania, and Philco were working on color CRT technology, they all had the same
problem: monochrome glass could not withstand the high voltages required to make
color workable. A breakthrough in glass additives solved this shortcoming and made it
possible to achieve 4 and 5 megapixel medical displays. The performance properties
and safety limits of the various glass melts are discussed in this book as they relate to
monochrome applications in medical imaging.

An overview of the architectural differences between color and monochrome CRTs
discusses how they are manufactured and the compromises required by their
respective design limits. This leads into the subject of electron optics. Here the main
focus is on monochrome optics because it provides performance beyond 1k-line
displays.

In addition to the performance of electron optics, the cathode (electron beam source) is
examined as a failure mode for all CRTs. The types of cathodes available and their life
expectancy are discussed in terms of cost of ownership, with an example calculation.
For medical applications, the inability to render full image fidelity is the true failure
mode, not the failure of the CRT to emit luminance. The way in which the electron
beam is formed and controlled through the optics determines the shape of the pixel and
thus the image quality. The influence of electron optics on the CRT gamma and related
performance compromises are discussed in conjunction with phosphor selection.

To say the CRT is a mature product is stating the obvious. Sir William Crookes
developed the progenitor of the modern electron gun in 1878 as he experimented with
variations on the Geisler discharge tube. Then in 1897, the German physicist Karl
Ferdinand Braun demonstrated a tube intended to display electrical waveforms. It was
not until 1920 that Vladimir Zworykin of Westinghouse Electric developed the other
components needed for the first camera and picture tube, respectively called the
iconoscope and kinescope.

In the 1930s the first broadcast architecture was tested using a format that became the
standard for North America. The National Television Standards Commission (NTSC)
established a format of 520 lines interlaced at 60 Hz refresh. Given the level of
performance available with vacuum tubes, interlacing the video with odd and even lines
was a necessity. In this way, the video amplifier wrote only half the lines with each
vertical scan. This in turn kept the horizontal scan rate down to 15 kHz. An NTSC
television, to this day, displays broadcast signals the same way it did in the late 1940s
when commercial television became a reality. Because of other limitations at the time,
only about 480 lines of the 520 in the signal can be seen. This is called overscan,
meaning that the active video is larger than the viewing space provided. Many control
problems could be hidden in the area just outside of what is visible.

Today's color monitors and monochrome medical-grade displays run at frequencies
well above those of television and put all the information within the available viewing
area. A SVGA boot format of 800 X 600 pixels starts at 30 kHz and climbs to 105
kHz to support 1600 X 1200 at 72 Hz refresh. In a medical portrait orientation, 118
kHz is required to support the same pixel format. A five megapixel (2560 lines) display
tops out at 180 kKHz. How does this differ from TV technology in terms of design
application? A TV set today can be reduced to a handfull of integrated circuits (ICs)
that include the power output for a number of circuits. The performance required for
medical-grade displays is not to be found in a handful of ICs.

Phosphors are more than just a color preference to be based on historical film usage.
There are efficacy and long-term aging considerations that determine calibration cycles
and the ability to color match old and new displays in multihead workstations. The
image quality as defined by the individual pixel requires careful consideration of both the
electron optics and phosphor performance. In addition, spatial noise is a factor to be
considered with all blended phosphors against the image complexity of the source
modality.

It is relatively easy to generate pixels. Being able to resolve them is the key to superior
image quality. The metrics involved in defining a pixel and how distortions can influence
the net results are illustrated using Microvision scans of both individual pixels and a
series of pixels at the Nyquist frequency with two types of optics and video amplifier.
Pixel fidelity is also separated into vertical and horizontal aspects of performance, which
are controlled by the optics and video amplifier, respectively, in a raster-scanned
device. This leads to a net performance as illustrated by a depth of modulation
(DMOD) scan for the same optic/video combinations.

Luminance uniformity on a CRT display is generally better than that of an average light
box. What contributes to this phenomenon is a multitude of events working against the
intended result. Compensation can be utilized, but there is a price to be paid, and how
CRT uniformity is defined is still subject to question. Two potential approaches to
defining uniformity are reviewed and weighed against uncompensated results.

Compliance with the DICOM grayscale standard display function (GSDF) is reviewed
to illustrate how important it is to specify a display's performance with hard numbers,
particularly video bandwidth. The background information from the preceding chapters
is needed to fully appreciate this discussion.

Test patterns and how to read them for information about a display's performance can
prevent second-guessing in the absence of test equipment. Use of the test pattern of the
Society of Motion Pictures and Television Engineers and Briggs test pattern 4 as quality
assurance tools illustrates the benefits of proper utilization and indicates what is not
acceptable. The American Association of Physicists in Medicine (AAPM), Task Group
18 has developed quality control patterns specifically for medical imaging.

The video card, whether it is a commercial graphics or a custom medical card, is part of
the video path and should always be tested in conjunction with the intended display.
Video card performance varies with manufacturer, and not all digital-to-analog
converters are created equal, all of which contribute to the shape of the pixel. Medical
grade cards are discussed with alternative paths using software preprocessing or
software compensation based on commercial color cards for monochrome displays.